1. Field of the Invention
The present invention relates to means to measure the elastic physical quantity of a sample in an imaging device which utilizes high-frequency acoustic energy, particularly an acoustic microscope.
2. Description of the Prior Art
A scanning acoustic microscope (abbreviated to "SAM") utilizing a very-high-frequency acoustic wave whose acoustic frequency is 1 GHz, accordingly whose acoustic wavelength in water is as short as approximately 1 micron, has been proposed in U.S. Pat. No. 4028933 by way of example.
The acoustic microscope produces an acoustic beam converging into a focal region of very small area by the use of an acoustic lens having a large numerical aperture F and mechanically scans a sample to-be-imaged which is held in and near the focal region, while it detects disturbed acoustic waves from the sample so as to obtain an image indicative of the distribution of elastic behaviors of the sample on the basis of the detection signal.
As one purpose of the acoustic microscope, it has been proposed to measure the elastic property of an observed sample. An acoustic beam having entered the sample is subject to such disturbances as reflection, refraction and transmission attenuation. The reflection of the beam is principally determined by the acoustic impedance of the sample, and the refraction and the attenuation are respectively determined by the velocity of sound through the sample and the attenuation constant of the sample. Accordingly, values indicative of the elastic properties of the sample can be obtained by measuring disturbed acoustic waves from the sample.
Such acoustic microscopes are generally of two types: the type which detects a reflected wave from a sample (the reflection type) and the type which detects an acoustic wave transmitted through a sample (the transmission type). The former can use a single transducer for both the generation and the detection of acoustic wave beams. In contrast, the latter requires individual transducers for the respective operations and needs to set these transducers at confocal positions, resulting in the disadvantage that the adjustment of the alignment of these transducers is difficult. Meanwhile, since the two phenomena of refraction and transmission attenuation of the acoustic wave beam by the sample pertain essentially to the transmission of the acoustic wave, measurements of the velocity of sound through the sample and the attenuation constant of the sample have been impossible with the prior-art acoustic microscope of the reflection type.
On the other hand, the transmission type acoustic microscope has been subject to the restriction that the velocity of sound and the attenuation constant through and of the sample can be measured only when the thickness of the sample is known.